Modern automotive finishes are generally composed of one or more primer layers and one or more top coat layers. The top coat layers often consist of a layer of base coat and a layer of clear coat. Each layer forms a distinct and measurable film. The quality of a painted part is generally indicated by the smoothness and uniformity of the finish and is commonly referred to as gloss. The level or amount of gloss on a finished part, referred to as its gloss index, requires a careful application of a precise amount of primer, base and clear coat. The minimum application thickness of base coat sufficient to mask the primer coat results in a higher gloss index than that achieved with a thicker base coat. The higher gloss index for a thin base coat has the advantage of providing an enhanced surface appearance and a cost savings in material for the finished article. Careful monitoring of the thickness of each layer of paint is required to achieve a high quality product.
2. Description of the Related Art
Previous methods of measuring a coating's thickness include various destructive means. A sample of the coated surface was sanded until the each layer was selectively removed. The thickness or each layer could then be measured.
Various nondestructive means for measuring a coating thickness also exist. Magnetic eddy current meters are capable of measuring the total thickness of a coating when applied to a ferrometallic substrate; but eddy current meters are incapable of measuring the individual thickness of a multi-layer coating Also, they cannot measure the thickness of a coating on a plastic or non-ferrometallic substrate.
Infrared absorption has been used to measure the thickness of water-based fillers on wood substrate as taught in U.S. Pat. No. 4,421,983 issued Dec. 20, 1983 to Fogle et al. These devices use the measured thickness of the filler to adjust the filler applicator. The amount of filler is controlled to maintain the coating weight or thickness within optimumal limits. Fogle et al. measures the thickness of each film sequentially. It does not measure the individual film thickness of an article having multiple layers previously applied.
Other devices use infrared radiation to measure the thickness of a coating on a metallic substrate. These devices use a black body radiator emitting beams of infrared radiation onto a coating applied to an infrared absorbing substrate. The infrared radiation emitted from both the coating and the substrate art compared to the radiation from the black body radiator and the total thickness of the coating is determined. These infrare devices require careful control of environmental factors especially temperature. The infrared radiation tends to warm the substrate, affecting the emitted radiation. These devices cannot measure the individual thickness of a multi-layer coating and they cannot measure the thickness of a coating on a nonmetallic substrate such as plastic because the nonmetallic substrate does not absorb sufficient infrared radiation.
Infrared absorption has been used to measure the thickness of a coating when applied to an infrared transparent film substrate. The infrared radiation penetrates both the coating and the film substrate and the amount of infrared absorption of the coating and film is measured opposite the infrared source. The film substrate must be relatively thin and made of a material which does not absorb or reflect infrared radiation. Such film substrates are generally not suitable for automotive painted structures such as fenders and doors. Such a method was taught in U.S. Pat. No. 4,631,408 issued Dec. 23, 1986 to Zelmanovic et al. Zelmanovic et al. teaches the use of infrared absorbance to measure the thickness and orientation of a flexible polymer such as polyethelene and nylon for use in bags. Zelmanovic et al. teaches passing an infrared beam of light through a multi-layer film and using the absorbance to calculate the thickness of each layer. When the film does not transmit sufficient light, absorbance can be measured by the reflected light. Zelmanovic et al. does not teach thickness measurement for solid films on rigid substrates, nor does Zelmanovic et al. teach a parabolic collecting mirror for the reflected light source.
It is an object of this invention to simultaneously measure the individual thicknesses of a multi-layer film applied to a substrate using near infrared radiation. The substrate is non-transparent to near infrared radiation and additionally maybe a non-reflective, nonmetallic substrate such as plastic. The film layers specifically include automotive paints such as base coat and clear coat layers.
It is another object of this invention to use the measured thickness of a film coating to control the application of film on a substrate.